The present invention concerns improvements relating to anti-collision warning lights and more specifically to a high-intensity anti-collision warning light and a method of driving the same for external use on aircraft.
In the field of anti-collision lights, there is a safety requirement to provide high-intensity regularly pulsed light on the exterior of an aircraft to enable the aircraft to be visible in all weather conditions. The intensity of the light is therefore quite high, typically being far greater than 100 Candela. Also, the flashing of the anti-collision light means that it is far more readily detectable than a light having a constant illumination. It should be noted that the flashing rate cannot be too high, namely above 25 Hz, because the light will be perceived by the human eye as being continuous. Furthermore, the intensity or intensity distribution has to be constant for each light flash.
Existing anti-collision lights, such as those described in U.S. Pat. Nos. 3,903,501 and 5,293,304, for example, use xenon flash tubes to generate the required intensity of light. The xenon flash tubes are driven by a discharge of an electrical capacitance into the flash tube, and so driving circuits include large banks of discharge capacitors. In addition, xenon flash tubes have high operating voltages that are generated in their driving circuits by transformers stepping up voltages to the required levels.
These requirements of often large, electrolytic capacitor banks and bulky transformers mean that the driving circuits tend to be large, heavy and expensive. This is particularly disadvantageous when several lights and their respective driving circuits are provided in an aircraft, where size and weight are very important issues. In addition, power consumption for these circuits can be undesirably high.
Conventional incandescent lamps that are used for aircraft navigation lights, for example, are far cheaper and require simpler, lighter driving circuits. However, incandescent lamps cannot generate the high intensity of light output required for warning lights.
It is desired to overcome at least some of the problems described above and to provide an alternative to existing high-intensity warning light technology.
The present invention resides in the appreciation that light emitting diodes can be used to replace xenon flash tubes in anti-collision warning lights for aircraft, and can be appropriately controlled to generate the required high-intensity light output. Under normal operation, light emitting diodes cannot generate the required light intensity levels, and previously this has mitigated against the use of light emitting diodes in high-intensity light output applications. However, the inventors of the present invention have determined that by overdriving a plurality of light emitting diodes with a pulsed control signal, the light output can be dramatically increased without overheating or otherwise damaging the light emitting diodes.
According to one aspect of the present invention, there is provided an anti-collision warning light for external use on an aircraft, the anti-collision warning light comprising: a light source having a plurality of light emitting diodes which are arranged to be pulsed with an overdriving signal to produce a higher than normal intensity flashing light output; and means for generating the overdriving signal, the signal comprising a sequence of drive pulses, each driving pulse having a magnitude sufficient to cause said relatively high-intensity flashing light output, the light source and the generating means being arranged such that in use the intensity of the generated light flashes is constant and is at least 100 Candela per flash.
The use of light emitting diodes obviates the need for large banks of capacitors and bulky transformers that are required for driving the xenon flash tubes. The driving circuit of the present invention can be realized in a simple control circuit that generates the required series of drive pulses at a relatively low voltage. Accordingly, the control circuit can be significantly smaller, lighter and cheaper than that of the prior art anti-collision warning lights and also has far lower power consumption. This latter aspect is particularly advantageous as aircraft lights are mostly operated from a rechargeable battery power supply.
Another significant advantage of using light emitting diodes in place of xenon flash tubes is that the light emitting diodes need to be replaced far less frequently than flash tubes. For example, xenon flash tubes last a few hundred flying hours, whereas light emitting diodes can last tens of thousands of flying hours. The longer operational life and greater reliability of light emitting diodes (light emitting diodes are less likely to malfunction than xenon flash tubes) can also provide significant cost savings in the long term. This is not only because of reduced costs of replacement components but more significantly because of the reduced costs of maintenance and/or labor. Furthermore, light emitting diodes are far more robust than flash tubes with far greater resistance to shock and vibration. For example, the light emitting diodes of the presently preferred embodiment of the invention, can withstand 5000 G of force and also random vibrations without breaking down.
Flash tubes also require shielding to prevent the electromagnetic radiation generated by the high-voltage transformers from affecting other equipment in the proximity of the warning light. A further advantage of light emitting diodes over flash tubes is that there is no requirement for this electromagnetic shielding, because light emitting diodes do not require high-voltage transformers.
The generating means can be provided on the light housing to provide a self-contained compact lighting unit. Also, the generating means is preferably configured to be operable in response to a received timing signal. The timing signal can be provided by an appropriate flash pattern box which can be positioned, for example, remotely from the anti-collision warning light. The flash pattern box typically includes a microprocessor and associated memory, which produce timing control signals at an appropriate rate, which are used by the generating means to cause the light source to output light pulses, which are perceived as regular intermittent illumination or as a regular sequence of light flashes.
The light emitting diodes are preferably set out in an array. This advantageously allows the light emitting diodes to be provided in a compact unit with a high light output density. In addition, the array may comprise a plurality of groups of light emitting diodes, the groups being connected together in parallel and each group comprising a plurality of light emitting diodes connected in series. This arrangement incorporates built-in redundancy, which advantageously prevents catastrophic failure of the light emitting diode array, because if one diode fails, the whole unit will not also fail. Rather, the series or string of diodes, which contains the defective light emitting diode, will fail and the other strings of diodes will be unaffected.
Preferably, the light emitting diodes are arranged to output a selected color of light which is dependent on the selected mode of operation. This may be achieved by the light emitting diodes comprising selectable sets of light emitting diodes, each set being capable of emitting a particular color of light. In this way, a single array of light emitting diodes, for example, can generate different warning signals depending on the color of the light output.
A further advantage of using light emitting diodes in place of xenon flash tubes is that light emitting diodes generate specific narrow wavelength bands of light and for military aircraft applications the amount of infra-red light that is generated can be accurately controlled. Accordingly, the use of light emitting diodes is inherently night vision goggle (NVG) friendly because it does not blind pilots flying on NVGs with excess infra-red light, and also there is no need to use special filter glass for reducing excess infra-red light from the warning light.
The warning light may comprise a plurality of light sources. This is advantageous when the warning light is to provide an increased field of view, in which case each source faces a different direction. For a maximal field of view, the plurality of light sources can be arranged about an axis to be radially outwardly facing and to illuminate substantially 360 degrees of view around the axis.
Preferably, the or at least one of the light sources comprises a plurality of infra-red light emitting diodes. The infra-red diodes provide a warning light which is detectable at night with NVGs. This is particularly for use on military aircraft during covert operations. The infra-red diodes are driven by a pulsed control signal in the same manner as the other light emitting diodes to generate the required high-intensity light output. When both infra-red and visible light sources are provided, the warning light can be switched between visible and covert operation modes. When viewed through NVGs the output of the infra-red light source appears identical to the visible light source. This aids night vision training because this type of training can advantageously be carried out in daylight conditions using NVGs.
Preferably, the light output from the warning light is flashed at a rate between 40 to 100 flashes per minute, namely 0.67 Hz to 1.67 Hz. This rate ensures that the output of the anti-collision warning light is relatively easy to detect visually by observers. Furthermore, this rate meets the requirements for international aviation authorities concerning the use of the anti-collision light on an aircraft.
The drive signals from the generating means to the light source are preferably in the form of a rectangular shaped waveform. This waveform provides an optimal light output over time from the light source as compared to other possible waveforms. The rectangular-shaped pulses preferably have a duty cycle of not more than 10%. This limit minimizes the power consumption for the warning light while still providing sufficient on time for generating the required intensity of light output.
In a presently preferred embodiment of the present invention, the set output of the light emitting diodes is controlled by the drive pulses of the overdriving signal being varied to adjust the driving current supplied to each of the light emitting diodes. However, it is also possible to control the light output by the drive pulses of the overdriving signal being varied to adjust the driving voltage applied to each of the light emitting diodes.
According to another aspect of the present invention, there is provided a method of driving a flashing anti-collision warning light for external use on an aircraft, the method comprising: pulsing a light source having a plurality of light emitting diodes with an overdriving signal to produce a higher than normal intensity flashing light output; and generating the overdriving signal, the signal comprising a sequence of drive pulses, each drive pulse having a magnitude sufficient to cause said relatively high-intensity flashing light output, the generating and pulsing steps being arranged such that in use the intensity of the generated light flashes is constant and is at least 100 Candela per flash.
According to another aspect of the present invention there is provided an anti-collision warning light as described above, in combination with a control means which is arranged to control a time sequence of light flashes output from the anti-collision warning light.